In radiological physics, charged-particle equilibrium (CPE) occurs when the number of
charged particles leaving a volume is equal to the number entering, for each energy and type of particle. When CPE exists in an irradiated medium, the
absorbed dose
Absorbed dose is a dose quantity which is the measure of the energy deposited in matter by ionizing radiation per unit mass. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection (reduction of harmf ...
in the volume is equal to the collision
kerma.
In order for this to occur, energy is needed.
Nuclear physics
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